At present, most of the drive circuits for the large-sized TFT-LCDs (Thin Film Transistor Liquid Crystal Display) are substantially developed by using the high-voltage (HV) CMOS (Complementary Metal Oxide Semiconductor) process. The gate drive circuit comprises a plurality of output channel circuits, wherein each output channel consists of a low-voltage logical combination circuit, a level shift circuit and a high-voltage output circuit. The advantages of the HV CMOS process are that, in the line(gate) drive circuit, the high-voltage output circuit of the output channel is simple, and the level shift is easily implemented.
FIG. 1 shows a level shift circuit in an output channel of a conventional line(gate) drive chip. As shown in the figure, VIN is an input drive signal; VOUT is an output drive signal; VCC is a low-potential power supply voltage; VGG/VEE is the high-potential voltage/ground. MP1-MP4 and MN1-MN4 constitute the level shift circuit, and MP5 and MN5 constitute the output drive circuit.
When HV MOS process is adopted, the high-voltage device in the output channel circuit has a large threshold voltage and a small transconductance, resulting in a large on-resistance, which limits the reduction in area of the chip and the increase in operating power of the chip. In contrast, the BCD process, which integrates the Bipolar devices, CMOS devices, and DMOS (double-diffused metal oxide semiconductor) devices, has the advantages of high integration, low power consumption, and small chip area, etc. It is particularly important that the LDMOS (lateral double-diffused metal oxide semiconductor) devices used in the BCD process have a large transconductance and a small on-resistance.
Compared with the high-voltage devices of the conventional HV CMOS process, the high-voltage devices of the BCD process have a small gate-source breakdown voltage. Therefore, during the application of the BCD process, the use of the above level shift circuit having the conventional structure as shown in FIG. 1 will cause the damages to the devices, and thus the functions of the chip cannot he normally achieved.
Therefore, anew structure for the level shift circuit is needed, so as to effectively avoid the occurrence of the phenomenon in which the high-voltage devices in the BCD process are broken down due to the excessively high gate voltage, and to stably realize the shift between the high and low voltage levels in the channel.